Modern cellular radio mobile communication systems may impose power efficiency and ruggedness specifications that cause exceptional technical problems to the designers of the RF transmission circuits. A typical example of these systems is GSM, where the mobile equipment is not allowed a permanent failure even under a severe load impedance mismatch. In most commercial GSM mobile equipment, worst-case mismatch conditions are determined by two customary system features.
First, no voltage regulator is used between the battery cells and the power amplifier. A higher than 2 watt output power level is required for GSM power amplifiers, which implies a strong current consumption. In such conditions, the use of a linear voltage regulator would result in excessive power losses due to large voltage drops, unless an application-specific regulator was properly tailored to the equipment. As far as efficiency is concerned, switching-mode voltage regulators offer better performance than linear ones, but the output power spectrum may be seriously compromised by spurious tones resulting from a residual ripple on the supply voltage. Therefore, direct connection of the battery cells to the power amplifier (PA) supply terminal has become the most commonly adopted approach for GSM systems. The drawback with this approach is that an oversupply condition occurs each time the mobile handset is put in the battery-charge state. In fact, a supply voltage as high as 5 V is provided to the power amplifier by fast battery chargers.
Moreover, no isolator is typically used between the PA and the antenna. As a result, the PA can incur strong load mismatch conditions due to antenna faults or disconnection. Power transistors should therefore be able to tolerate substantially increased overvoltage stress, as collector voltage waveforms show much higher peaks under mismatch conditions than in nominal operation.
The worst-case conditions occur when the power amplifier is subject both to oversupply and load mismatch. For this reason, the ruggedness specification is usually expressed in terms of a maximum tolerable output VSWR under a specified oversupply condition. Typical data sheets of commercial power amplifiers guarantee that no permanent damage is caused by a load VSWR of 10:1 under a supply voltage of 5 V. Simulated collector waveforms are shown in FIGS. 1 and 2 for a 34 dBm output power stage under nominal (VCC=3.5 V, matched load) and worst-case (VCC=5V, 10:1 load VSWR) operating conditions, respectively. Considerable collector voltage peaks as high as 20 V are attained, indicating that active devices with a very high breakdown voltage are needed.
The maximum operating region of silicon bipolar transistors has recently been investigated and their breakdown behavior experimentally compared to GaAs heterojunction bipolar transistors (HBTs) as disclosed in A. Inoue, S. Nakatsuka, R. Hattori, and Y. Matsuda, “The maximum operating region in SiGe HBTs for RF power amplifiers,” in IEEE MTT-S Int. Microwave Symp. Dig., June 2002, pp. 1023-1026. The ability of silicon bipolar devices to operate with collector peaks beyond the nominal (dc) breakdown voltage has been demonstrated, whereas GaAs HBTs cannot survive such a test. Despite this, too high breakdown voltages (i.e., more than three times the maximum supply voltage) are still required for silicon BJTs to comply with commercial ruggedness specifications. This is quite a difficult challenge, especially for transistors optimized to attain best power performance in low-voltage operation. Indeed, breakdown voltage enhancements are only possible by reducing the doping level and/or increasing the thickness of the collector layer, which has a detrimental effect on the power-added efficiency (briefly PAE) due to greater losses on the collector series resistance. Therefore, some kind of circuit arrangement may be needed to improve device ruggedness.
Circuital approaches have been proposed in literature, which allow VSWR protection by clamping the voltage peaks either at the collector or base of the power transistor. In K. Yamamoto et al., “A 3.2-V operation single-chip dual-band AlGaAs/GaAs HBT MMIC power amplifier with active feedback circuit technique,” IEEE J. Solid-State Circuits, vol. 35, pp. 1109-1120, August 2000, a VBE multiplier is effectively used to limit the collector voltage swing, as shown in FIG. 3. When the collector-base voltage of the power transistor exceeds the turn-on voltage of the VBE multiplier, a large current flows through the protection circuit and the collector voltage is prevented from reaching the breakdown limit. Conversely, in J. R. King, “High VSWR mismatch output stage,” U.S. Pat. No. 6,137,366, Oct. 24, 2000, a VBE multiplier senses the base-emitter voltage of the power transistor, as shown in FIG. 4. A portion of the base current is absorbed by the additional circuitry when the base-emitter voltage exceeds a certain threshold. Therefore, the collector current of the active device is limited and secondary breakdown is avoided.
The use of clamping protection circuits may result in RF performance degradation. Indeed, base voltage clamping limits full input overdrive of the power transistor, whereas collector clamping hampers a proper waveform shaping through harmonic manipulation as disclosed in F. H. Raab, “Class-F power amplifiers with maximally flat waveforms,” IEEE Trans. Microwave Theory Tech., vol. 45, pp. 2007-2012, November 1997. Input overdrive and waveform shaping are both crucial to achieve high-PAE operation.
A 30 dB transmit power control range may be needed in the GSM system to optimize the propagation link budget. In most commercial GSM mobile equipment the power control is performed by a closed-loop approach as disclosed in P. Cusinato, “Gain/bandwidth programmable PA control loop for GSM/GPRS quad-band cellular handsets,” IEEE J. Solid-State Circuits, vol. 39, pp. 960-966, June 2004. As shown in FIG. 6, the RF power is sensed at the amplifier output using a directional coupler and is detected by a diode. The resulting signal is compared to a reference voltage through an error amplifier whose output drives the PA gain control terminal. The loop forces the sensed voltage and the reference voltage to be equal. Therefore, power control can be accomplished by changing the reference voltage.
The main drawback of the closed-loop approach is the output power loss due to the directional coupler, which strongly affects the overall efficiency of the transmit chain. Moreover, the loop gain varies considerably over the dynamic range because of the nonlinear control characteristic of the PA. As a consequence, the control of power regulation transients may be made difficult and the system may even suffer from stability problems.
An alternative approach has been proposed in D. Brunel et al., “Power control with the MRFIC0913 GaAs IPA and MC33169 support IC,” Motorola Semiconductors application note AN1599, 2000. Available online at: http://e-www.motorola.com/files/rf_if/doc/app_note/AN1599.pdf. The reference discloses an approach which acts on the PA's supply voltage to control the output power of the amplifier, as shown in FIG. 6. By using a simple linear regulator (implemented through a power PMOS transistor and an operational amplifier), the PA collector voltage can be linearly varied through a control terminal. Reducing the collector voltage effectively limits the output swing and, hence, the power delivered. This technique allows for excellent power-setting accuracy without the need for closed-loop output power sensing. The implementation of a tailored supply voltage regulator (sometimes embedded in the PA module) eliminates the need for the complex and lossy external circuitry required for a closed-loop approach, including the directional coupler, power detector, and error amplifier. However, a very low RDS(ON) (not higher than a few tens of milliohms) is needed for the PMOS transistor to preserve the overall efficiency under maximum output power conditions. Therefore, an extra-cost is entailed for the use of such a high-performance device.
Another drawback impairs the above-mentioned open-loop approach. A nearly linear relationship between the collector dc voltage and the RF output voltage is obtained when regulating the power supply level. This is particularly true for FET-based switching PAs (e.g., Class E amplifiers as disclosed in N. O. Sokal and A. D. Sokal, “Class E—A new class of high-efficiency tuned single-ended switching power amplifiers,” IEEE J. Solid-State Circuits, vol. SC-10, pp. 168-176, June 1975). Such a linear control characteristic turns to logarithmic when expressed in dB/V units. It thus suffers from a very steep control slope at low power levels, which calls for an increased resolution of the digital-to-analog converter driving the gain control terminal.
The document EP-1,387,485-A1 discloses an RF power amplifier including an external control loop and a protection circuit. In the external loop a collector current from the output transistor is detected and regulated in respect to a reference current. The protection circuit detects a voltage envelope at the collector electrode of the output transistor and utilizes this signal for a bias reduction signal that is input to the base electrode of the driver transistor.
Another closed-loop approach to limit the output of the last stage of an RF power amplifier to protect it against high voltages that could lead to a destructive breakdown of the output power transistor is disclosed in the document WO 03/034586 A1. The collector voltage of the output transistor is sensed through an AC coupled resistive voltage divider and compared with a threshold value for eventually reducing the base current of the output power transistor. A control voltage Vcontrol commands both the turning on of a biasing circuit of the output power transistor and a switch for isolating the collector voltage sensing network to prevent undue dissipation.
A drawback of this approach may be the inability to adjust the threshold that determines the intervention of the attenuating control in function of the detected peak levels of the output voltage (the collector voltage of the output power transistor). Moreover this approach exploits a control loop, which is always locked. This could be a potential source of instability, especially in modern wide-band applications.
Moreover overall energy efficiency may be enhanced by switching off all current generators with a single external command to eliminate any current absorption when the amplifier is in an off state without employing a dedicated switch for isolating the output sensing network.